1. Field of the Invention
The present invention relates to a process for preparing a thin film composed of monomoleclar layers. More particularly, it relates to a process which is similar to Molecular Beam Epitaxy (MBE) technique, Atomic Layer Epitaxy (ALE) technique or Molecular Layer Epitaxy (MLE) technique for glowing a series of monomolecular layers on a substrate in a predetermined order.
The process according to the present invention is preferably applicable to a preparation of a superconducting thin film composed of compound oxide.
2. Description of the Related Art
Molecular beam epitaxy (MBE) technique is a kind of vacuum deposition processes and is used for manufacturing a thin film of GaAs, AlAs or the like in the field of semiconductor devices. Atomic or molecular layer epitaxy (ALE or MLE) technique is also proposed for preparing a thin film of GaAs, AlAs or the like by Junichy NISHIZAWA (see Nippon Ketsho Gakaishi (28) 1986, pp. 133-141). Metal-organic vapour phase deposition (MOCVD) technique is also proposed to prepare a thin film of single crystal. In this technique, organo-metals such as trimethylgallium and arsine are heat-decomposed on a substrate and chemically bonded to a lower layer. The decomposition of organometals may be effected also by photon (see Applied Physics Letter (49) 13 pp. 706-710, May/June 1986).
The superconductivity is a phenomenon which is explained to be a phenomenon of a kind of phase change of electrons under which the electric resistance become zero and the perfect dimagnetism is observed. Thus, under the superconducting condition, electric current of a very high current density can be delivered without any loss of power. The superconducting materials can be used in the fields of electronics, for example, as a device using the Josephson effect in which quantum efficiency is observed macroscopically when an electric current is passed through a weak junction arranged between two superconductors. Tunnel junction type Josephson device which is a typical application of the Josephson effect is expected to be a high-speed and low-power consuming switching device owing to a smaller energy gap of the superconducting material. It is also expected to utilize the Josephson device as a high sensitive sensors or detectors for sensing very weak magnetic field, microwave, radiant ray or the like since variation of electromagnetic wave or magnetic field is reflected in variation of Josephson effect and can be observed as a quantum phenomenon precisely. Development of the superconducting devices is also demanded in the field of high-speed computers in which the power consumption per unit area is reaching to the upper limit of the cooling capacity with increment of the integration density in order to reduce energy consumption.
However, the critical temperature of superconductivity could not exceed 23.2K of Nb.sub.3 Ge in which was the highest Tc for the past ten years. The possibility of an existence of new types of superconducting materials having much higher Tc was revealed by Bednorz and Muller, who discovered a new oxide type superconductor in 1986 [Z. Phys. B64 (1986) 189].
The new type compound oxide superconductor discovered by Bednorz and Muller is represented by [La, Sr].sub.2 CuO.sub.4 which is called K.sub.2 NiF.sub.4 -type oxide having a crystal structure which is similar to known perovskite type oxide. The K.sub.2 NiF.sub.4 -type oxides show such higher Tc as 30K, which are extremely higher than known superconducting materials. It was also reported that C. W. Chu et al discovered, in the United States of America, another superconducting material so called YBCO type represented by YBa.sub.2 Cu.sub.3 O.sub.7-x having the critical temperature of about 90K in February 1987. Still other type new superconducting materials which were reported recently are a compound oxide of Bi--Sr--Ca--Cu--O system and Tl--Ba--Ca--Cu--O system which exhibit such high Tc as more than 100K and which are chemically much stable than the abovementioned YBCO type compound oxide or the like. And hence, the possibility of existence of high-temperature superconductors have burst onto the scene.
A thin film of the above-mentioned new type superconducting material can be prepared by the sputtering technique in which metal elements which are components of the superconducting compound oxide are sputtered and deposited in oxygen atmosphere onto a substrate.
Persons skilled in the art may conceive such an ideal that the MBE technique is the most suitable process for preparing the superconducting thin film of the superconducting compound oxide, since a series of monomolecular layers can be deposited in a predetermined order by the MBE technique.
However, the conventional MBE technique can not apply directly to a preparation of a thin film of the new type superconducting compound oxide, because a crystal of the compound oxide has a plurality of layers each of which contains different elements. Namely, in the conventional techniques of MBE, ALE and MLE, each mono-atom layer to be deposited is composed of only one element to realize a well-ordered crystal structure. For example, in the case when a film film of GaAs composed of two elements is prepared by the conventional MBE technique, these two elements are deposited independently and alternately on a substrate. In other words, these two elements can not be deposited simultaneously.
Still more, a main theme of study in the conventional MBE have been directed to how to control the deposition rate of each layer, in other words, how to terminate each deposition when a monomolecular layer is realized. Furthermore, little study have been done for preparing a thin film of compound oxides because of above-mentioned reasons and also because it is difficult to deposit a monomolecular layer containing oxygen atoms.
An object of the present invention is to provide a process for preparing a the thin film of compound oxide and composed of layered or stratified mono-layers each having a different composition by the MBE technique.
Another object of the present invention to apply the process for a production of a superconducting thin film of the new type compound oxide by the MBE technique.